In the manufacture of web based products, such as paper, textiles and certain plastics, a web of material is moved along a serpentine path through various stations wherein a different manufacturing operation is performed on the web at each station. A web moving through such a path can measure several hundred feet in length and can measure several feet in width. Should the web break during the manufacturing process, significant downtime can occur while the web is rethreaded through the different stations. As will be appreciated, such downtime can result in substantial cost to the manufacturer. An additional consequence of a web break is the detrimental effect on product quality if breaks are occurring too frequently.
Therefore, a need exists in the manufacture of web based products for methods and apparatus for preventing web breaks. One invention directed towards this problem is described and claimed in a copending application entitled, METHOD AND APPARATUS FOR DETERMINING WEB FLUTTER, Ser. No. 192,255, filed May 10, 1988, owned by the assignee of the present application and incorporated herein by reference. While that invention was primarily concerned with the detection of web flutter and the production of a signal representative of the amplitude and frequency of such flutter, the present invention applies that signal in an apparatus and method for the suppression of web flutter and thus the prevention of web breaks.
Flutter is that phenomenon where the web moves in a direction substantially perpendicular to the direction of travel, which movement has one or more amplitudes and frequencies. Since touching the web during production is to be avoided, if possible, it would be desirable to detect and control web flutter in a fashion which does not contact the web. While copending application Ser. No. 192,255 detects web flutter in a non-contact fashion, the present invention suppresses web flutter in a non-contact fashion.
Devices have been previously disclosed for the determination only of web flutter in a non-contact fashion, while flutter suppression was attempted through direct contact with the web. U.S. Pat. No. 4,496,428--Wells and related U.S. Pat. No. 4,501,642--Wells discuss the use of reflected light in order to determine the amplitude and frequency of web flutter during paper manufacture. Such information is thereafter used to change the movement or location of various rollers, i.e. modification of the web drive roller velocity or the reciprocation of a piston connected web contacting roller, so that tension in the paper web can be maintained at some desired level. Although this patent suggests the use of radar or ultrasonic devices for determining flutter, no method or apparatus is disclosed.
U.S. Pat. No. 4,637,727--Ahola et al. also discusses the use of light to make a non-contact determination of web flutter, however, no discussion appears as to how such flutter could be controlled. It is said in that patent that the minimization of flutter results in the probability of a web break being smaller. Basically, it appears that flutter amplitude and frequency are determined through the use of a high frequency distance measuring scheme. A light pulse is reflected off a moving paper web and directed onto a photodiode. The time it takes the light to travel from its source to the photodiode is measured. Over a period of time, sufficient measurements can be made to determine the frequency and amplitude of web flutter. This patent also suggests the use of capacitance or ultrasound to determine web flutter; however, for different reasons each of these techniques is rejected in favor of the light based technique.
One problem with these previously described devices is that they do not appear to be practical in the manufacturing environment. For example, in the manufacture of paper it will be necessary to determine flutter within web pockets where temperatures can reach 180.degree. F. or higher. Also, if a web break occurs in or around the region where flutter is being determined the device being used can be struck either by the advancing web or by the end of the web, i.e. the break tail. The forces involved in such contacts can be significant enough to damage light based devices.
It is also a practice in web manufacturing that if flutter appears to be too severe such that a web break or excessive wrinkling is feared, the flutter is reduced by slowing the movement of the web through the machinery. As will be appreciated, the slowing of the web results in the manufacturing equipment being operated at less than capacity, which is economically undesirable.
Consequently, a need exists for method and apparatus to control web flutter in a manufacturing environment in a non-contact fashion.